This invention relates in general to video systems and, in particular, to video systems including a Teletext (TTX) processor and a television receiver including an on-screen display capability.
There is a growing trend in more expensive television receivers (TVs) toward replacing conventional switches that activate and control certain TV functions with microprocessor control means that are responsive to viewer initiated commands received from a keyboard, that may be either on the TV or remotely located. Such receivers may include "on-screen" displays to apprise the viewer of the functions and of their status. Thus, special features, such as automatic telephone dialing and selection of signal sources, as well as conventional features, such as control of channel and volume and control of color, tint and brightness of the picture may be implmented in a microprocessor based TV. Generally the function and features to be controlled and their status are selectable by the viewer and displayed on a portion of the TV screen. A very common form of on-screen display briefly indicates the channel number to which the TV is tuned and the time of day when the TV is turned on or when a channel change is made. The display may also be actuated when the viewer operates a special key on the keyboard.
Another service that is growing in popularity is that known as Teletext (TTX) wherein alphanumeric and graphic displays are transmitted on certain lines of the vertical blanking intervals of a conventional television signal. The transmitted information is digitally encoded and not visible to a viewer without a special TTX processor. The encoded information is acquired by the processor and stored in a dynamic random access memory (DRAM) that is coupled to a character generator which provides the appropriate video output signals for displaying a selected TTX "page" on th TV screen. Each TTX page consists of 40 characters across and 24 rows of characters down. TTX systems are well known in the art and reference may be had to U.S. Pat. No. 4,393,404 for a data acquisition circuit that is responsive to viewer commands for acquiring a selected page or pages of the recycling TTX information and for storing the 40.times.24 character array of digitally encoded data comprising each page. This stored, encoded data is applied to a display generator which decodes the stored information for synthesizing R, G and B character signals which drive the cathode ray tube (CRT) of the TV for producing a display reflecting the stored data. The display thus comprises a page of 24 rows of text or graphics, with each row consisting of up to 40 characters.
In television receivers having on-screen message display capability, the microprocessor control unit is assigned the task of generating the characters for producing the on-screen display. The microprocessor also performs many other functions, such as tuning, audio adjustment, color control and production of timing signals in addition to generating the on-screen messages. The invention enables a significant simplification in the microprocessor software by delegating the generation of at least some of the on-screen messages to the TTX processor. This is accomplished by incorporating the messages in a read only memory (ROM) that is addressable by the microprocessor as pages of TTX data.